ATM APS is described in the 1.630 specification of the International Telecommunications Union (ITU-T). ATM APS provides a mechanism for quick recovery from physical faults, akin to APS mechanisms used in synchronous optical networks (SONETs). The protected entity in this case is an ATM VP or VC circuit. A description of ATM APS as specified by ITU-T specification 1.630 will aid in understanding the invention.
Referring to FIG. 1, consider an ATM network with arbitrary topology, and an ATM circuit (either VPC or VCC) extending from a node A to a node B, where node A is connected to a switching system SA and node B is connected to a switching system SB. In the context of traffic flow from A to B, the protection domain for this circuit extends from SA to SB. SA is sometimes referred to as the source for the protection domain of the circuit. SB is sometimes referred to as the sink for the protection domain of the circuit. Other nodes on the paths between source and sink are sometimes referred to as intermediate nodes along the protected circuit.
ITU-T specification 1.630 describes two different types of protection, so-called “1+1” protection, in which the source transmits (multicasts) the traffic constantly on both paths and the sink selects at any time one of the incoming traffic streams and forwards it to the egress, and so-called “1:1” protection, in which the source transmits the traffic on only one of the paths. When that path becomes disconnected, the source switches to transmit on the alternate path. An intermediate node that senses a problem on its associated path notifies the sink node using the AIS operations and maintenance (OAM) cell. The sink then effects a protection switch (1+1 protection) or notifies the source that the source should switch to the alternate path (1:1 protection).